1. Field of the Invention
The invention relates to a vehicle steering apparatus having a steering wheel operated by a driver and a rear wheel-side steering driving mechanism that steers rear wheels in accordance with the operation of the steering wheel.
2. Description of the Related Art
Japanese Patent Application Publication No. 7-291138 (JP-A-7-291138), for instance, discloses a vehicle steering apparatus in which energization of an electric motor that drives a rear wheel steering mechanism is discontinued, and the rear wheels are restored to a steering neutral position by way of a centering spring, when vehicle speed drops to “0”.
Japanese Patent Application Publication No. 2001-80537 (JP-A-2001-80537), for instance, discloses a four-wheel steering apparatus in which rear wheels are not steered, by keeping a rear wheel power cylinder in a non-output state, while a vehicle remains stopped, even if a front wheel steering mechanism is operated.
Japanese Patent Application Publication No. 6-122377 (JP-A-6-122377), for instance, discloses a rear wheel apparatus in which a torque accumulating means, for accumulating torsional force in the rotation direction, is provided on a rotary shaft that transmits input revolutions from front wheel power steering to rear wheel power steering, such that the revolutions of a rear rotary shaft that is provided more rearward than the torque accumulating means are restricted when the vehicle is stopped, and are unrestricted when the vehicle starts moving.
In four-wheel steering vehicles where the rear wheels can be steered, the rear wheels are ordinarily kept in a neutral steering position or, alternatively, steering of the rear wheels is not allowed, when the vehicle is stopped, as disclosed in JP-A-7-291138, JP-A-2001-80537 and JP-A-6-122377.
However, in the vehicle steering apparatus disclosed in, for instance, JP-A-7-291138, the rear wheels are returned to the neutral steering position by a centering spring when the vehicle is stopped. This results in a more complex rear wheel steering mechanism. During vehicle cruising, moreover, the electric motor must steer the rear wheels against the elastic force of the centering spring at all times. The output of the electric motor must be accordingly greater, which may result in a larger electric motor size and in increased power consumption.
In the four-wheel steering apparatus disclosed in JP-A-2001-80537 and the rear wheel steering apparatus disclosed in JP-A-6-122377, moreover, steering of the rear wheels is not allowed when the vehicle is stopped, regardless of the steering of the front wheels (or by way of operation of the steering wheel). As a result, the rear wheels remain steered to a position removed from the neutral steering position when, for instance, the vehicle is stopped, even if the front wheels have returned to the neutral steering position. This may be unaesthetic and may cause the vehicle to meander for an instant when the vehicle starts moving again. Preferably, therefore, the rear wheels as well should be returned to the neutral steering position when the vehicle is stopped.